The invention is directed to stretch forming material, and particularly to controlled stretch forming of material such as ductile materials used in modern aircraft to form ducts, external skins, and the like.
Mechanical tattle tales have been used for stretch forming for a great number of years and define the present state of the art as known today.
Widely known state of the art tattle tales, an example of which is shown in FIG. 1 and identified as prior art, consists of a generally rectangular shaped piece of foil or thin material B having a longitudinal axis of greater length than width. Equally spaced apart graduations C are inscribed or printed on the upper portion of the rectangular foil or material to indicate specific distances therealong. A pair of vertical cuts D are located on each longitudinal end of the rectangular portion. An elongated tattle tale recorder strip E is inserted within the vertical cuts D as shown so as to allow relative movement between the rectangular and elongated portions. When the elongated strip E is inserted through the cuts, an arrow indicator F pointing in the direction of the graduations is located so that the arrow direction indicates the end graduations adjacent to the rectangular shaped foil's attachment point to the material. The end H of the elongated strip, remote from the rectangular attachment point, is attached to the material to be stretched and the material is then stretched until the arrow on the elongated portion aligns with the desired graduation on the rectangular portion. In a typical application, the arrow is moved slightly further than required to allow for any spring back elasticity in the material to insure the exact stretch length desired.
Generally the material to be stretch formed is located remote from the stretch forming machine and the machine operator. This requires that at least a second worker trained in the stretch forming art be present to watch the tattle tale and signal the machine operator to terminate the machine stretch operation when the desired length of stretch has been accomplished.
The current employment and method of use of state of the art tattle tales depends on the expertise of the machine operator and a second person viewing the tattle tale to communicate at the proper time so that the delay between the signaled termination time and the machine operator's reaction time to that signal and machine termination occurs at the proper time. Over stretching will occur at times regardless of the conscientiousness of the people performing the stretch forming operation.
There has not been a simple and accurate means to perform exact and repeatable stretch forming of material until the emergence of the present invention.